Characterization of electrochemical deposition of copper and copper(I) oxide on the carbon nanotubes coated stainless steel substrates

The nanocomposite coatings composed of carbon nanotubes and various forms of copper were prepared in the two-step process. Firstly, carbon nanotubes were coated on stainless steel substrate using electrophoretic deposition at constant current. Then, the process of electrochemical deposition using copper(II) sulphate solutions was performed under high overpotential conditions. The modification of the copper(II) cations concentration in the solution and the deposition time provided the formation of various forms of crystals. The samples and their cross-sections were observed and examined using scanning electron microscope equipped with electron dispersive spectroscopy system. The analysis of chemical composition revealed that in addition to the pure copper crystals, the crystals characterized by the presence of copper and oxygen were formed. Therefore, Raman spectroscopy was applied to determine the unknown stoichiometry of this copper oxide. The point and in-depth analysis identified copper(I) oxide in the form of different size crystals depending on the concentration of the copper(II) sulphate solution. To confirm these findings, grazing incidence X-ray diffraction measurements were also performed. the combination of the applied methods has provided the detailed description of the preparation of the nanocomposite coatings with the proposed mechanism of copper(I) oxide formation.


Characterization of electrochemical deposition of copper and copper(I) oxide on the carbon nanotubes coated stainless steel substrates Jakub Marchewka * , Ewa Kołodziejczyk , Patryk Bezkosty & Maciej Sitarz
The nanocomposite coatings composed of carbon nanotubes and various forms of copper were prepared in the two-step process. Firstly, carbon nanotubes were coated on stainless steel substrate using electrophoretic deposition at constant current. Then, the process of electrochemical deposition using copper(II) sulphate solutions was performed under high overpotential conditions. The modification of the copper(II) cations concentration in the solution and the deposition time provided the formation of various forms of crystals. The samples and their cross-sections were observed and examined using scanning electron microscope equipped with electron dispersive spectroscopy system. The analysis of chemical composition revealed that in addition to the pure copper crystals, the crystals characterized by the presence of copper and oxygen were formed. Therefore, Raman spectroscopy was applied to determine the unknown stoichiometry of this copper oxide. The point and in-depth analysis identified copper(I) oxide in the form of different size crystals depending on the concentration of the copper(II) sulphate solution. To confirm these findings, grazing incidence X-ray diffraction measurements were also performed. the combination of the applied methods has provided the detailed description of the preparation of the nanocomposite coatings with the proposed mechanism of copper(I) oxide formation.
Electrochemical methods are still considered one of the most favorable techniques of production of composite coatings, despite the continuous development of other methods [such as physical vapor deposition (PVD) or chemical vapor deposition (CVD)]. They stand out from the rest because of their high versatility, repeatability and simplicity in combination with precise control over the properties of obtained products 1,2 . The other advantages are low cost, reduced amount of waste materials and easy scalability of the equipment used in electrochemistry processes. Moreover, by changing the process parameters, it is possible to tailor the crucial properties of coatings such as their thickness, roughness and morphology [3][4][5] . These advantages, together with variety of received materials and their applications place electrochemistry in the spotlight for researchers from many fields of science. Composite coatings, wherein the particles in the conductive matrix could be metallic, polymeric or ceramic have been used successfully in electronics, surface engineering, aerospace or corrosion protection [6][7][8][9][10][11] . Electrochemical deposition (ECD) processes have been known since the beginning of the nineteenth century, but research is still ongoing to explain their mechanisms. Although, electrochemical reactions occurring during the electrodeposition process are relatively easy to balance by several redox equations, the individual steps of the process which run according to specific mechanisms are still the subject of research and process modeling 12,13 . To put it simply, the ECD is based on the modification of the conductive substrate surface with a thin and tight adherent coating of desired material deposited from solution. It takes place at the interface of the two phases: liquid (electrolyte) and solid (electrodes) in the closed electrical circuit. The system could be far from the state of chemical equilibrium, because the applied potentials may differ from the equilibrium values determined by the Nernst equation or the Pourbaix diagrams 14 . Therefore, by controlling the applied potential and pH, it is possible to obtain various forms (chemical, structural or crystallographic) of material from the same solution [15][16][17] .
Copper is one of the metals most widely used in the industry and currently most applied material in ECD. This is mainly due to its excellent thermal and electrical conductivity and anti-corrosion properties. In acidic solutions, copper(II) cations (Cu 2+ ) are directly reduced to metallic copper (Cu) according to the following reaction:  GIXRD measurements of the samples. Phase composition of the samples was characterized by grazing incidence X-ray diffraction (GIXRD) measurements using Empyrean (Malvern Panalytical, United Kingdom) diffractometer with CuKα1 line applied as a radiation source and Ge(111) as a monochromator. The scans were collected applying θ-2θ geometry in 2θ range from 10 to 90° and 0.001° step size with an incident angle of 1.0°. The data was processed using Highscore Plus 3.0 software with PDF-4+ database.

Results and discussion
The scope of the tests. The samples were obtained in the form of CNT layers on the stainless steel plates modified by the process of ECD using CuSO 4 solutions. Their examination was focused on the microstructure and composition of the electrodeposited material in relation to the parameters of ECD process. Significantly different concentrations of CuSO 4 solutions, i.e. 1 mM and 100 mM, as well as high overpotential conditions were intentionally applied. A general overview of the samples with particular attention to the form of the electrodeposited material on the surface was carried out using SEM. This method was also applied for the cross sections of the selected samples to examine the microstructure of CNT layers. During this analysis, the elemental composition was evaluated by EDS with a special emphasis on copper and oxide. Two samples (A7 and B7) prepared by the longest running process of ECD (600 s) were additionally investigated in detail as the most complex systems in terms of the microstructure. Raman spectroscopy was applied to examine the distinctive forms of the electrodeposited material using the point and in-depth analysis. To confirm the results of the tests the GIXRD measurements were also performed.
Material electrodeposition in terms of the process parameters. The initially applied EPD method enables the preparation of the tight CNT coatings on the stainless steel substrates. In the process of ECD two regions for the material electrodeposition could be distinguished. The first includes the surface of CNT layer whereas the second one is its volume. Therefore, they were analyzed separately in this research. At the beginning, the selected and applied ECD process parameters should be considered. All samples were prepared in high overpotential conditions. It is known [43][44][45] that this parameter is critical for the reduction rate of metal cations in similar systems. In order to understand this phenomenon two parts of the solution volume could be distinguished, i.e. a thin interface directly in the vicinity of a substrate and a bulk solution. When a low overpotential is applied Cu 2+ cations are relatively slow depleted from the near substrate layer. Consequently, their concentration is easily replenished and maintained by their diffusion from the bulk solution. Crystal formation is limited by the process of Cu 2+ cations reduction and driven by the minimization of surface energy. Contrary, when high overpotential is applied the deficiency of Cu 2+ cations in the proximity of the substrate is relatively rapidly generated. Therefore, their diffusion rate from the bulk solution is not enough to maintain their concentration www.nature.com/scientificreports/ in this volume. Crystal growth is limited by the process of mass transport. It could be also mentioned that high overpotential conditions in water solutions promote the evolution of hydrogen on an electrode [46][47][48] . This effect is important when the aim is to prepare a tight layer of electrodeposited material on a substrate. As a consequence of the production of gas bubbles and their diffusion from the substrate surface to the bulk solution the cavities are formed. It was not observed for the prepared samples, because the duration of ECD process was too short to obtain a high surface coverage by the electrodeposited material. As it is described in the Pourbaix diagram for Cu 26,27 the potential which provides the reduction of Cu 2+ cations to metal Cu is about hundreds of mV depending on the pH of the solution. Therefore, in this research high overpotential conditions were applied. This results in the domination of the mass transport limited mechanism of the crystals formation.
Characterization of the samples surface by SEM. Two series of the samples were prepared in the process of ECD using the different concentration of CuSO 4 solutions, i.e. 1 mM for A1-A7 and 100 mM for B1-B7. Hence, with the application of the same high overpotential the concentration of Cu 2+ cations play a major role in the mechanism of the crystals growth. As it is shown in the SEM images of A7 and B7 samples (Figs. 1, 2) a significantly different crystal structures formed on CNT layers. Using 1 mM solutions in the process of ECD the dendritic crystals were obtained (Fig. 3). These structures are typically formed in a local non-equilibrium state [49][50][51][52] . Hence, in this case the typical mechanism of crystal growth limited by the mass transport may be assumed, as it was explained before. Contrary, using 100 mM CuSO 4 solutions the spherical crystals agglomerates were produced (Fig. 4). Well-defined single crystals are formed when the mechanism of their growth is driven entirely by the kinetics of the process of cations reduction. The observation of the agglomerates could indicate the existence of mixed mechanism limited partially by both the mass transport as well as the Cu 2+ cations reduction. The effect resulting from the application of high overpotential conditions was partially reduced by a significantly faster diffusion rate of Cu 2+ cations from the bulk solution to the interface. This was provided by the significantly higher concentration of Cu 2+ cations used for the preparation of B series samples in comparison to A series samples.
To examine the crystal growth two series of the samples were prepared. The SEM images (Figs. S1-S7 and Figs. S8-S14, respectively, in supplementary data) reveal their gradual formation. For A series samples the first well-defined dendrites were observed after 120 s of the electrodeposition (Fig. S2). With the increasing duration of the process, they progressive growth was revealed and the surface coverage was higher. The examination of B series samples shows a significantly different picture. The formation of the spherical agglomerates is already noticed after 60 s of the electrodeposition (Fig. S8). They are growing bigger with time. The other effect is the formation of crystals within CNT layer. At the beginning they are small and could be noticed as its local inhomogeneity (Fig. S8), but with the increasing duration of the process they are also growing bigger. Finally, some of these crystals get a spherical shape, but the other obtain a well-defined octahedral shape (Figs. 5, 6). It was also found that after 600 s of the electrodeposition the octahedral crystals grow to the surface of CNT layer.
Analysis of the crystals structure by Raman spectroscopy. Raman spectroscopy was used to examine the formation of crystals on CNT layer (Fig. 7). For this, the single point measurements were performed for B7 sample in the characteristic points chosen using confocal microscopy (Fig. 8). The first one is positioned on pure CNT layer (point A) whereas the others correspond to the different crystal structures, including the spherical agglomerate which partially grew to the surface (point B) and the octahedral crystal on the surface (point C).   53,54 . Two the most intense bands are located at 1357 cm −1 (so called D band) and at 1587 cm −1 (so called G band). The first one is related to the presence of disorder in sp 2 carbon structure (A 1g mode) whereas the second one is associated with the C-C vibrations, both along the CNT axis and circumferential (E 2g mode). The other weak band observed at about 2710 cm −1 is a second-order harmonic of the D mode (so called 2D band) which could be attributed to the three dimensional long range order of CNT 55,56 . The third spectrum (for point C) was obtained for the octahedral crystal on the surface. A set of bands typical for Cu 2 O at the range from 150 to 650 cm −1 are found. This could be confirmed by the comparison (Table 2) with the spectra obtained for pure copper oxides (Fig. S15, in supplementary data). The most intense band at 647 cm −1 is associated with the T 1u (LO) mode. The other ones at 151 cm −1 and 219 cm −1 are attributed to the T 1u (TO) and 2E u modes, respectively. Finally, the weak bands at 417 cm −1 and 501 cm −1 could be related to the four-phonon 3E u + T 1u (TO) mode and to the T 2g mode, accordingly. Therefore, this assignment proves that the observed octahedral crystals are composed of Cu 2 O 57, 58 . In case of the measurement for point B, the spectrum is a result of the probing depth characteristic of Raman spectroscopy. The bands at 1366 cm −1 ,         www.nature.com/scientificreports/ Figure 11 shows the exemplary SEM images for the cross sections of A7 sample. CNT layer with the dendritic Cu crystals on the surface could be identified (Fig. 11a). In its entire volume the nanometric crystals are visible. They are also present in the fragments without Cu deposits on the surface (Fig. 11b). The distributions of the chosen elements obtained by EDS for the cross section of A7 sample are presented in Fig. 12. Chromium and iron are the main components of the stainless steel substrate (Fig. 12d,e) whereas nickel and gold were introduced as the additional layers in preparation for the analysis (Fig. 12h,i). The distribution of copper and oxygen (Fig. 12c,f) within CNT layer confirms the presence of copper oxides and the previously described Raman measurements indicated that this is Cu 2 O. Some small share of the oxygen content could be also associated with the functional groups of CNT introduced during their initial modification.
The exemplary SEM images for the cross sections of B7 sample are shown in Fig. 13. In the entire volume of CNT layer, the crystals with dimensions of hundreds of nanometers are observed (Fig. 13a). Therefore, they are significantly bigger than the ones noticed in A7 samples. Some large agglomerates within CNT layer which partially grow to its surface are also identified (Fig. 13b). The others could be formed on the surface or entirely in the volume of CNT layer (Fig. 14a). The distributions of the chosen elements obtained by EDS for the cross section of B7 sample are presented in Fig. 14. The origin of chromium and iron (Fig. 14d,e) as well as nickel and gold (Fig. 14h,i) is the same as for A7 sample. The distribution of copper and oxygen (Fig. 14c,f) within CNT layer also confirms the presence of copper oxides in form of the larger nanometric crystals. Based on the previously described Raman measurements it is known that this is Cu 2 O. Careful analysis of the oxygen distribution also reveals that the large agglomerates are composed only of Cu, but the one on the surface additionally shows the traces of surface oxidation. This result is in accordance with the confocal microscope image (Fig. 7) and the Raman analysis in point B (Fig. 8) which in the similar situation indicates the presence of some amount of Cu 2 O. In the SEM image a local inhomogeneity of CNT layer is observed and explained as related to the crystals growing in its volume. The octahedral Cu 2 O and spherical Cu agglomerates are formed and could grow to the surface. It is known 41,60 that the thermodynamic deposition potential of metal cations on the same metal substrate is lower in comparison to the one for the substrate composed of the other materials. This is due to the formation of the same continuous crystal lattice by the electrodeposited substance. This effect could be the explanation for the observed Cu agglomerate which grows on the other one partially located within CNT layer (Fig. 14a). When Cu agglomerate grows to the surface it provides a preferred site for the Cu deposition in comparison with pure CNT layer.

Examination of the CNT layer composition be GIXRD. To support the findings of the SEM-EDS and
Raman analysis the GIXRD measurements of the A7 and B7 samples were performed (Fig. 15). This method allows to assess the surface composition of the layers because of the limited penetration depth of X-rays. The diffraction peaks assigned to Cu (COD 4105681) and CNT (COD 1566075) could be identified in both diffractograms, but the ones attributed to Cu 2 O (COD 1000063) may be observed only for B7 sample (Table 3). This is consistent with the previous results and confirms the presence of the octahedral Cu 2 O crystals which grow to the surface of CNT layer during the process of ECD using 100 mM CuSO 4 solution.
The mechanism of copper(I) oxide formation within CNT layers. Raman and SEM-EDS analysis revealed the formation of Cu 2 O within CNT layer during the process of ECD. For the samples prepared in the process of ECD using lower concentration of CuSO 4 solution, i.e. 1 mM (A series), the nanometric crystals were formed whereas using higher concentration of the solution, i.e. 100 mM (B series), the crystals with dimensions of hundreds of nanometers were grown. To explain this effect, we propose the mechanism based on the reaction:  CNT in the layer are functionalized by the initial introduction of the negatively charged functional groups (mainly carboxyl COO − groups). Some of Cu 2+ cations which diffuse from the bulk solution to their proximity are electrically attracted by them. This bonding enables only the partial reduction of Cu + cations instead of the total reduction to metal Cu. These cations remain bonded to the functionalized CNT. If their local concentration reaches a specific threshold and all the functional groups are occupied, then the crystallization of Cu 2 O occurs. In case of the lower concentration of CuSO 4 solution, i.e. 1 mM, the diffusion rate significantly decrease. Therefore,

Conclusions
The high overpotential conditions and the significantly different concentrations of CuSO 4 solutions applied in the ECD process enabled the formation of various forms of crystals on the surface and within the volume of CNT layer. This indicates that by changing these parameters, the microstructure and chemical composition of the electrodeposited material could be easily controlled. SEM-EDS and Raman spectroscopy, with the addition of GIXRD to confirm the results obtained, were shown as a complete set of methods providing detailed characterization of the samples prepared in this way. Specific attention was given to the formation of crystals within the CNT layer. This required in-depth Raman spectroscopy measurements and SEM-EDS analysis for the cross sections. The study showed that in the high overpotential conditions and using CuSO 4 solutions with a concentration of 1 mM the formation of dendritic crystals is favored. However, at their concentration of 100 mM, well-defined single crystals are formed. Such differences could be explained by various mechanisms involving the diffusion and reduction during the ECD process. At the lower concentrations of the solutions crystal growth is limited by mass transport, and at the higher concentrations-by the kinetics of the process of cations reduction. A noteworthy phenomenon that was observed for the samples prepared was the formation of Cu 2 O crystals. A mechanism for this process was proposed based on the electrostatic bonding of Cu 2+ cations by the negatively charged functional groups of CNT, which results only in their partial reduction to Cu + cations. When their local concentration reaches a certain threshold, the crystallization of Cu 2 O occurs. The results obtained may be supportive in the preparation of composite materials based on Cu and CNT with the designed microstructure easily controlled by the ECD process parameters.